Unicellular protozoan cilite, Stentor coeruleus, is able to sense differences in light intensity and wavelength, the organism being most sensitive to red light (610 nm); Stentor also senses the direction of light propagation by exhibiting light-avoiding and negative phototactic swimming behaviors, respectively. This aneural photosensory phenomenon is triggered by the photoreceptor, and our proposed work involves elucidation of how the photoreceptor perceives light signal and processes it for subsequent sensory transduction of the light energy into behavioral responses. In particular, we propose to study the possible involvement of light-induced proton release from the photoreceptor as a primary mechanism of light signal perception processing.